A rocket propulsion system is used to lift and propel spacecraft with a payload into space. The rocket propulsion system includes a rocket engine powered by rocket engine propellant to produce a gaseous exhaust providing necessary thrust to lift the spacecraft into orbit. A rocket engine includes a rocket motor, a combustion chamber and an exhaust nozzle. Rocket engine propellant is burned in the combustion chamber to produce the exhaust that may exit through the nozzle. The rocket propulsion system produces thrust by generating pressures in the combustion chamber and nozzle. The exhaust thrust pushes and hence lifts the launch vehicle off the surface of the earth into orbit.
One type of rocket engine nozzle is the bell shaped rocket nozzle having a bell shaped exit cone. The bell shaped exit cone may be configured to a predetermined size with a predetermined expansion ratio. That is, the ratio of the diameter of the nozzle at the entrance of the nozzle at the combustion chamber to the diameter of the nozzle at the aft end where the exhaust gases exit the nozzle. For a launch off the ground, extremely high expansion ratio nozzles are not employed because the exhaust separates from the nozzle wall causing large side forces during launch. When the nozzle size is over expanded with a high expansion ratio, then the nozzle exit pressure may be less than the local atmospheric pressure and a resulting portion of the nozzle is producing negative thrust as a drag effect. On the other hand, when the nozzle is under expanded with a low expansion ratio, then the nozzle exit pressure is greater than the local atmospheric pressure and the nozzle may not be producing as much thrust as would a larger highly expanded nozzle. Optimum thrust production occurs when the nozzle is perfectly variably expanded so that the exit pressure just matches the atmospheric pressure that changes with altitude during the launch phase.
Consequently, as a launch vehicle increases in altitude during the launch phase, the optimal outlet pressure of the nozzle exhaust should change as the atmospheric pressure decreases. None of the currently used rocket boosters change the outlet pressure of the respective nozzles during the launch phase. Typically, boosters use a fixed sized nozzle with an outlet pressure that is selected to optimize the average performance during the launch phase. These booster nozzles typically over expand the exhaust gases at liftoff and under expand the exhaust gases at high altitudes. One type of rocket engine is the Aerospike engine that is the only current rocket engine in development that uses a variably sized nozzle for maximizing lift thrust during the entire launch phase through launch altitude levels. Rocket engines lift performance is optimized by variably sized nozzles through mechanical enhancements. Continuously optimizing the exit pressure of a nozzle has the potential to greatly increase the performance of the rocket engine.
Referring to FIG. 1, the potential increase in lifting capability for a typical booster is maximized when the expansion ratio is matched to the upper dashed curve ending with an expansion ratio of ε=5200:1. The upper dashed performance line ending with the ratio ε=5200:1 is the specific impulse of the engine when the nozzle is optimized at all altitudes. The lower solid performance line shows the specific impulse with a fixed ε=8:1 expansion ratio of the nozzle with engine performance matched to low altitudes with a lower performance at higher altitudes. The middle dashed performance line shows the specific impulse when the nozzle is fixed to a ε=32:1 expansion ratio. The ε=32:1 expansion ratio yields a lower performance at low altitudes because nozzle is over expanded but has a higher performance at higher altitudes. Hence, it is desirable to vary the expansion ratio of a nozzle by varying the nozzle exit diameter during the launch phase. The nozzle exit diameter could be expanded by a factor of two during the launch phase for increasing the expansion ratio by a factor of four, for example, from ε=8:1 to ε=32:1. The optimum specific impulse would be given by the optimized dashed line up to 40K feet. After reaching an altitude of 40K feet, the value of the specific impulse would follow the ε=32:1 performance line for the remainder of the launch phase flight. Even this limited variability would increase the specific impulse from 309 seconds to 337 seconds.
The specific impulse function of a rocket engine has an effect on a payload lift capability of a rocket. The payload delivered to orbit is a function of the average specific impulse of the engine. The launch path during the launch phase can be calculated using the orbital parameters. Large payload capability gains can occur from increases in the specific impulse of the rocket engine. This potential payload lift capability gain can be realized using techniques for continuously optimizing the nozzle exit pressure of the rocket engine.
The advantages of continuously optimizing the outlet pressure of a rocket nozzle during the launch phase flight have been known for fifty years over which time many mechanical designs have been proposed. Unfortunately, all of those mechanical designs have been considered too costly, too heavy, or too complicated to be incorporated in a practical launch vehicle. Large heavy complicated mechanical systems that variably adjust the exit diameter of the nozzle will suffer from increased costs and reduced reliability of the rocket propulsion system. These and other disadvantages are solved or reduced using the invention.